Knowing the orientation of the relative wind is necessary for piloting an aircraft. This knowledge makes it possible to compute an incidence parameter, from which flows the computation of a critical parameter: the lift. Knowing the lift of the aircraft, at every moment during the flight, is absolutely necessary to the safety of the flight. This knowledge is provided partly by a wind vane. The wind vane is an element attached to the skin of the aircraft. It comprises a rotating base and a profiled plate that is oriented in the direction of the wind. The rotating base and the plate are connected by a join. They are for example made of metal alloy. The wind vane is connected so as to pivot about an axis orthogonal to a plane tangential to the skin of the aircraft. Like any aeronautical probe, the wind vane, and in particular its plate, must be heated in order to withstand the icing conditions encountered during flights. Icing deforms the calibrated outer surface of the plate, unbalancing it, and therefore falsifying the measurement of the orientation of the relative wind and consequently the computation of lift associated with this orientation.
In order to ensure the de-icing of the wind vane, an elongated heating element, called a heater, is placed inside the wind vane. This heater, from the inside of the wind vane, heats the outer surfaces of the wind vane. The heater can be inserted either via a trailing edge of the plate, or via the rotating base of the wind vane. The heater comprises ceramic blocks forming a heating resistor. The blocks are surrounded by two conductive plates forming electrodes for the heater. The conductive plates are installed, brazed or bonded to the ceramic blocks. For safety reasons, the outer surface of the wind vane must not be subjected to an electric potential. The ceramic blocks and the conductive plates are consequently electrically insulated from the plate and the rotating base by a polymer or resin coating or by an electrically insulating film.
Such an electrical insulation of the ceramic blocks and of the conductive plates has several drawbacks. A first drawback is that this electric insulation also forms a heat barrier between the heating ceramic blocks and the plate of the wind vane. To compensate for this loss of heat energy transmission, the ceramic blocks must operate at very high temperatures. These high temperatures impose design constraints, notably in the choice of the polymer, of the resin or of the insulating film which must withstand these temperatures, and in the mechanical connection between the conductive plates and the ceramic blocks. These high temperatures also involve a considerable electricity consumption and, in certain cases, a premature aging of the ceramic blocks.
One drawback, associated with the use of a polymer or resin coating, is that this coating is secured to the ceramic blocks and the conductive plates, while the materials forming these various elements have very different expansion coefficients. These differences cause stresses in the mechanical connections between the polymer or resin coating and the ceramic blocks. These stresses become greater as the operating temperatures rise. The result of this is a fatigue phenomenon that is due to the sequence of thermal cycles and is capable of culminating in the breakage of the mechanical connections. In the event of breakage, even partial, the electric insulation may fail, the outer surface of the wind vane consequently being subjected to an electric potential. Similarly, the connection by installing, brazing or bonding between the conductive plates and the ceramic blocks is subjected to stresses due to the differential expansion that can result in the breakage of the electric connection between the conductive plates and the ceramic blocks. Such a breakage prevents the heating and therefore the de-icing of the wind vane. The computation of the incidence and consequently of the lift is then falsified.
Another drawback, still associated with the use of a polymer or resin coating, is that this coating must be applied uniformly; otherwise there is the risk that electrical contacts will occur between the electrodes and the plate. The uniformity of the coating is dependent on many parameters, in particular on the composition and the temperature of the coating when it is applied.
Another drawback of the electrically insulating film is that, although in principle it has a uniform thickness, there is the risk that it is damaged when the heater is inserted into the plate. The plate can then be subjected to an electric potential.